1. Field of the Invention
The present invention relates to a piezoelectric vibration device utilizing a thickness-shear vibration available for communication devices or the like. In giving particulars, the present invention relates to a piezoelectric vibration device which can restrain influences by a spurious vibration or an unnecessary vibration of the general non-harmonic overtone mode, and enhance guarantee attenuation characteristics.
2. Description of the Prior Art
Referring to FIGS. 31 and 32, prior art is described as below.
FIG. 31 is a diagram of an electrode structure and a vibrational energy distribution of spurious vibration of non-harmonic overtone mode, generally referred to as a (3,1,3) mode, seen from a bottom surface of a conventional piezoelectric vibration device. FIG. 32 is a sectional view along line S--S in FIG. 31 and a diagram of the vibrational energy distribution of spurious vibration of a principal vibration mode (substantial line) and the (3,1,3) mode. Each symbol of (3,1,3) designates an order of overtone in a Y'-axis (thickness) direction, an order of overtone in an X'-axis direction, and an order of overtone in a Z'-axis direction. As shown in FIG. 31, the (3,1,3) mode is a mode of the spurious vibration having three vibration crests (troughs) arranged in parallel to the Z'-axis direction.
As a quartz plate 90, an AT cut quartz plate is employed. A principal plane of the quartz plate 90 is provided with an input electrode 191, an output electrode 192, outgoing electrodes 191a, 192a for leading the input electrode 191 and the output electrode 192 to peripheral portion of the quartz plate 90, and the other principal plane of the quartz plate 90 is provided with a common electrode 193, and an outgoing electrode 193a for leading the common electrode 193 to the peripheral portion of the quartz plate 90. Each electrode is formed by means of thin film forming means using vacuum evaporation method or the like. Though it is not shown, the input and output electrodes 191, 192 are respectively connected to an input terminal and an output terminal, the common electrode 193 is hermetically confined in a state of being connected to a grounding terminal.
In the piezoelectric vibration device having such a structure, the input and output electrodes 191, 192 formed oppositely each other in a state wherein the quartz plate 90 intervenes therebetween and the common electrode 193 are designated as resonant areas, a symmetrical mode (fs) and oblique symmetrical mode vibration (fa) are confined by the electrodes, they are acoustically coupled thereby forming a multiple mode principal vibration with the result that the piezoelectric vibration device having pass band characteristics as a predetermined filter is designed.
Moreover, in the prior art, as high-frequency type or three-order overtone type electrode materials, aluminium is generally employed.
As mentioned above, the piezoelectric vibration device is designed so that, as well as the principal vibration, a higher-order non-harmonic overtone mode such as the spurious vibration of the (3,1,3) mode is confined by the electrodes, thus exciting them simultaneously. Furthermore, as well: as such a mode, there are various spurious vibrations such as one referred as to a (3,3,1) mode having three crests (or troughs) of vibration arranged in parallel in the X-axis direction, or a vibration mode having three crests (or troughs) of vibration respectively in the axis direction. These spurious vibrations wherein energy is confined under exciting electrodes, thus exciting the vibration as a standing wave relatively strongly, have problems wherein spurious vibrations affect the principal vibration, the spurious vibrations disorders the pass band characteristics, and the guarantee attenuation characteristics in portions outside of the pass band are not obtained enough. In case of employing aluminium as the electrode material in the same way as prior art, it is easy to oxidize aluminium, thus resulting in bad yield and productivity. In addition, in case that aluminium oxide is formed by oxidizing aluminium, a conductive resistance to a conductive adhesive is greater so that the guarantee attenuation characteristics are inferior.